This country's increasing dependence upon foreign sources of petroleum and natural gas has caused considerable consternation. Since native resources are dwindling precipitously, increased efforts must be made, in view of our vast coal fields, to move away from petroleum by-products and natural gas to coal derivatives. However, many industries cannot be readily transformed from natural gas to coal directly without incurring prohibitive costs. In order to avoid these enormous changeover costs, attempts have been made to economically convert coal into a natural gas substitute. This coal derivative has been denominated synthetic natural gas (SNG) to distinguish it from natural gas obtained at the well head.
Various methods of coal gasification are well known in the art. The two more common procedures are the moving bed gasification process, exemplified by the conventional Lurgi dry bottom and the British Gas Co. (BGC) Lurgi slagging gasifier, and the entrained bed gasification process such as the Koppers-Totzek or the Texaco Partial Oxidation Process.
In the moving bed process, the bulk of the gasification is carried out at a relatively low temperature of about 1200.degree.-1600.degree. F. in the coal bed. In this temperature range, a substantial amount of methane is formed which becomes part of the product along with methane subsequently produced in later process steps. This methane produced in the gasifier is called primary methane. Final gasification at the bottom of the bed can be carried out at about 1600.degree. F. in the coal bed if the ash is to be below fusion temperature (like conventional Lurgi) or above the flow temperature of molten ash (about 2400.degree.-2800.degree. F.) as British Gas Corp Lurgi Slagging Process.
By heating the coal to the gasification temperature (1200.degree.-1600.degree. F.), devolatilization of the coal takes place which results in co-production of tar, oil, phenol, ammonia, HCN and many organic components. These components leave the gasifier along with the crude synthesis gas and undecomposed steam. After cooling the synthesis gas, a phenolic liquid containing dissolved phenol, NH.sub.3, HCN, CO.sub.2, H.sub.2 S is rejected from the process and needs to be treated in order to become environmentally accepted as a waste effluent. This problem is particularly severe in the conventional non-slagging moving bed process because the gasifier bottom, due to process requirements, is kept below the ash fusion temperature which calls for a very large steam injection in order to quench the gasifier bottom zone. This results in a very large quantity of unconverted steam in the gasifier overhead, and consequently very large quantities of phenolic condensate must be treated. Both slagging and non-slagging moving bed processes need sized coal (usually 1/4"-2" range). In the sizing process, about 10-35% of the coal will be reduced to a size smaller than 1/4" and cannot be gasified. This fine coal is usually used as a boiler fuel.
If a slagging moving bed process is employed, the steam requirement for gasification is greatly reduced as compared with non-slagging processes because no steam is needed to quench the bottom zone. As a result, the thermal efficiency of the process is higher and the amount of phenolic condensate to be treated is reduced. The treatment of the phenolic condensate includes phenol extraction, ammonia recovery, and bio-treatment. Because the steam requirement is greatly reduced, coal fines that otherwise could be used as a boiler fuel can no longer be used.
If entrained bed gasification is used on a stand-alone basis, SNG production may not be economical. The high temperature of gasification (2200.degree.-2700.degree. F.) will lead to a very large oxygen requirement, very low primary methane formation, and consequently, to a very large shift methanation unit and a relatively low H.sub.2 S/CO.sub.2 ratio in the gas to be desulfurized (i.e., selective H.sub.2 S recovery is probably required which is more expensive). The overall thermal efficiency of coal to SNG by entrained bed gasification is lower than moving bed, and the capital cost is higher. The advantage of the entrained bed is that no tar, oil, phenol and ammonia by-products are produced, and consequently, environmental problems are greatly reduced.